The dynamic pressure on stator surfaces at vane tip

mid-span and root was measured to demonstrate the flow structure in a compressor working close to its stability limit.Unsteady pressure signals show that before stall there exist two kinds of pressure disturbances.The frequencies of these disturbances are approximately in proportion to rotor speed and the relative frequencies are 5 and 8 respectively in terms of rotor rotating frequency.The intensity of the disturbance with relative frequency 5 increases as mass flow decreases until compressor working point reaches stall margin

whilst the intensity of other disturbances increases first and drops suddenly as compressor works at stall point.Thus

the flow field disturbance which leads to rotating stall/surge should be that the relative frequency is 5 and it can be regarded as the warning signal of instability.Setting a value λ as the threshold

establishing a reliable and effective stall avoidance control based on pressure signals detected by high response transducers seems to be promising.